Can anyone advise what adhesive and technique to use to repair a short section (~6 cm ~2.5 inches) of seam that has come apart on our spinnaker, in an interior seam ~155 cm (~61 inches) from the head?

Is the spinnaker cloth material nylon?

It appears that the panels were attached to each other at the seams via a strip of adhesive ~12 cm (slightly less than 0.5 inch) wide, after which the seams were sewn with the large zig-zag stitch. The sewing doesn't appear to contribute very much to holding the seams together, though.

It seems likely that the adhesive was applied via transfer tape. A web search for same turned up Innotec of Wisconsin. Their sales rep suggested their TH253 transfer tape, which I understand they sell for making spinnakers and will cut to width as ordered and ship in small quantities to retail customers for a fairly reasonable price.

For surface preparation, I plan to use isopropyl alcohol. I'm concerned that any stronger solvent, such as acetone (polar) or mineral spirits (non-polar) might be harmful to the material.

I used Silcone co-polmer sealant for use on glass aluminium etc. Use the one that will work for Aquariums, not all Silcone co-polmer sealant are the same. The spin is made from silcone impregment fabric, nothing else will stick.

Just smear it onto the patch or seam and spread with a blade or tool and press together - goes off in a few hours and full strength in a few days. Its very strong. Make sure the sealant is not lumpy when applied, it needs to be nice and smooth.

Most sail lofts use "seam stick" tape on dacron or nylon sails prior to sewing. It's a double sided tape available in various widths, thicknesses and adhesive levels. Chutes usually get the really thin stuff. And the seam stick can be like contact cement - you better get it down right the first time.

I use the double sided tape only to fix the two parts of the spinnaker and then go over it with my sewing machine (zig zag will do). If there is no tape to fix the cloth you will end up with two badly aligned sides at the end of the repair. ThatÅ› why sailmakers use this trick.

I decided to go with my original plan, and it worked out very well. Here's a recap in case it will be useful to others:

I had to repair a short section (~6 cm ~2.5 inches) of seam that came apart on our spinnaker, in an interior seam ~155 cm (~61 inches) from the head.

It appears that the spinnaker panels were attached to each other at the seams via a strip of adhesive ~12 cm (slightly less than 0.5 inch) wide, after which the seams were sewn with the large stitches in a zig-zag pattern. The adhesive appears to provide essentially all of the shear strength to hold the panels together while the spinnaker is filled; the sewing just keeps the layers from peeling apart during handling.

The seams show no evidence of any tape backing, only adhesive. Thus, it seems apparent that the adhesive was applied via transfer tape. A web search for same turned up Innotec of Wisconsin. Their sales rep suggested their TH253 transfer tape, which I understand they sell for making spinnakers. They cut it to width as requested (12 mm v. 0.5 in. = 12.7 mm in stock), and mailed a roll to me for a very reasonable price. I think 3M might make a comparable product, but I was not able to find anyone to sell a small quantity retail, never mind custom width.

For surface preparation, I used isopropyl alcohol (IPA). I'm concerned that any stronger solvent, such as acetone (polar) or mineral spirits (non-polar) might be harmful to the material.

I cut a piece of the adhesive transfer tape to the size & shape of the gap, then cut it in half to facilitate positioning the ends. I then used tweezers & popsicle sticks to position the tape, rollered with a clean wall paper seam roller to make sure the adhesive was stuck down, removed the backing with the tweezers, positioned the top layer, & re-rollered.

Then my wife re-stitched the seam by hand, using polyester thread. Biggest problem: The adhesive kept gumming up the needle, requiring frequent cleaning with IPA.

The repair has held up fine so far, through ~5 hours in largely ~15 knot winds. This was with a fair amount of flogging, shrimping & getting hung up on the bow, which we plan to reduce as we gain experience.

So, all in all, I'm happy with the repair, except for the amount of time it took to figure out how to do it! I probably spent half a day researching the subject. But, with the benefit of experience, I think it would take ~1 hour do to a similar repair again, less timethan to take it to a sail loft unless there is one near by, & certainly less expense.

Yes, Claus was correct - the seams still have to be sewn. Sorry I didn't mention it earlier. I just assumed it was understood.

As a follow on...

I think most sail lofts will sell single rolls of seam stick relatively cheaply (we will). Another source is RC sailboat shops. Hard core RC sailboaters are known for building their own sails using seamstick.